If you are finding that your credits have disappeared, check if Auto Assign is on. If it is then your credits were just assigned to your websites automatically for you. You can see if it is on or off in the Websites page, above your first website.

Autoassign runs every hour, so your credits will build up for a bit and then be assigned to your websites. This can make it appear they were randomly deleted. You can turn it off if you'd rather assign the credits yourself!